Can inflationary axion fluctuations dominate over misalignment?
Abstract
The relic abundance of light stable axion dark matter is usually set by misalignment. We study whether the inflationary fluctuations of the axion can dominate instead, and we follow the modes analytically through an instantaneous transition from de Sitter expansion to radiation domination, neglecting the axion mass during inflation and keeping it afterward. Since the energy density is quadratic in the field, a fluctuation-dominated abundance carries an isocurvature perturbation even when the mean displacement vanishes. For a canonical minimally coupled axion the spectrum is scale invariant, and fluctuation-dominated dark matter exceeds the Planck isocurvature limit by about eight orders of magnitude. An inflaton-dependent kinetic function or a nonminimal coupling to the Ricci scalar tilts the spectrum blue, moving the fluctuation power from CMB scales to small scales. Imposing the Planck limit at the pivot scale, we find that fluctuation-dominated dark matter with a constant kinetic slope requires the spectral tilt nη−1≳0.27 and the axion mass mη ≳200 GeV at HI=1010GeV, and nη−1≳0.34 and mη≳4.5keV at HI=1013GeV. A kinetic function with a running slope lowers these masses to 1.4 GeV and 0.7eV, respectively. For the QCD axion, whose oscillations start late, we estimate that any single power-law spectrum blue enough to satisfy the pivot limit gives too little fluctuation abundance, and misalignment dominates.
Related
- Axion Misalignment Across First-Order Phase Transitions
- QCD axion misalignment during reheating
- The axion mass in the post-inflationary scenario in a minimal scaling model
- From quantum correlations to inflationary tracking scalar field evolution
- Induced field-space curvature from a heavy inflationary trough
BibTeX
@misc{lee2026inflationary,
title = {{Can inflationary axion fluctuations dominate over misalignment?}},
author = {Seung J. Lee and Sarunas Verner and Bingrong Yu},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.36033},
eprint = {2610.36033},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.36033},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Can inflationary axion fluctuations dominate over misalignment?
AU - Seung J. Lee
AU - Sarunas Verner
AU - Bingrong Yu
PY - 2026
DA - 2026/10/26
PB - arXiv
DO - 10.48550/arxiv.2610.36033
UR - https://arxiv.org/abs/2610.36033
AB - The relic abundance of light stable axion dark matter is usually set by misalignment. We study whether the inflationary fluctuations of the axion can dominate instead, and we follow the modes analytically through an instantaneous transition from de Sitter expansion to radiation domination, neglecting the axion mass during inflation and keeping it afterward. Since the energy density is quadratic in the field, a fluctuation-dominated abundance carries an isocurvature perturbation even when the mean displacement vanishes. For a canonical minimally coupled axion the spectrum is scale invariant, and fluctuation-dominated dark matter exceeds the Planck isocurvature limit by about eight orders of magnitude. An inflaton-dependent kinetic function or a nonminimal coupling to the Ricci scalar tilts the spectrum blue, moving the fluctuation power from CMB scales to small scales. Imposing the Planck limit at the pivot scale, we find that fluctuation-dominated dark matter with a constant kinetic slope requires the spectral tilt n_η−1≳0.27 and the axion mass m_η ≳200 GeV at H_I=10^10GeV, and n_η−1≳0.34 and m_η≳4.5keV at H_I=10^13GeV. A kinetic function with a running slope lowers these masses to 1.4 GeV and 0.7eV, respectively. For the QCD axion, whose oscillations start late, we estimate that any single power-law spectrum blue enough to satisfy the pivot limit gives too little fluctuation abundance, and misalignment dominates.
ER -